The present invention is related to a method of cleaning and removing a liquid from a surface of a substrate by means of a rotating cleaning pad. Said method can be used in the fabrication process of integrated circuits, especially when the preceding processing step has heavily contaminated the surface.
In general, the complete and efficient removal of a liquid from a surface of a substrate is a multiply repeated step in e.g. the fabrication process of integrated circuits. Such a step can be performed after a wet etching step or a wet cleaning step or a wet rinsing step or any other step used in said fabrication process wherein said substrate is treated or exposed or immersed in a liquid. Said substrate can be a semiconductor wafer or a part thereof or a glass slice or any other slice of an insulating or conductive material.
The manufacturing of integrated circuits evolves towards processing of each substrate individually rather than in batches of several substrates. In the state of the art of IC manufacturing, most processing steps, such as implantation steps, deposition steps, are already performed in a single substrate mode. On the other hand, wet processing steps, such as cleaning steps and subsequent liquid removal steps, are typically performed in a batch mode because of a lack of appropriate alternatives. Therefore, differences in waiting times are created for each individual substrate between a wet processing step, performed in a batch mode and another processing step, performed in a single substrate mode. Such variability is undesirable with regard to process control. Moreover this mixed batch and single substrate processing increases the cycle time, which is also undesirable. Therefore, there is a general interest in the development of competitive single substrate wet processing steps. Particularly, one of the major challenges regarding single wafer wet processing is a method for removing a liquid from both sides of a substrate. There are two major requirements to be fulfilled for such a method. First, it should work sufficiently fast. In state of the art production lines, a substrate is processed typically every 2 to 3 minutes. Ideally, in order to avoid equipment duplication, the liquid removal should be completed in about such a time frame. Another requirement is related to the preferred orientation of the substrate. State of the art processing equipment and transportation tools are developed to handle substrates in a horizontal position. Therefore in order to avoid additional substrate handling, it would be desirable to perform the wet processing steps using horizontally positioned substrates.
In the European Patent EP 0 385 536 B1, a method is disclosed of drying substrates after treatment in a liquid by pulling said substrate slowly out of said liquid. However, to be effective, this known method, which is based on the Marangoni principle, requires that the substrates are pulled out of the liquid in an upright position, i.e. a surface of said substrate is about perpendicular to the surface of the liquid bath (as can be seen in FIGS. 1 to 6 of the European Patent EP 0 385 536 B1). This handling is incompatible with the majority of the other process steps where the equipment and transportation tools are developed to handle horizontal positioned substrates.
In the U.S. Pat. No. 5,271,774, a spin-drying technique is disclosed which is able to handle horizontal positioned substrates. In fact, several small liquid islands are formed being removed from the substrate by a rotary movement. It is known that such a spin-drying technique leaves undesirable residues, often referred to as drying marks, on the substrate surface, particularly on surfaces having mixed hydrophilic and hydrophobic regions. Furthermore, this technique is not suited to remove residual contamination in the form of particles.
It is clear from the above discussion that there is a general interest in single wafer wet processing and drying. Therefore, an efficient drying technique which also has good cleaning capabilities is a prerequisite. For some inherently dirty processes such as chemical mechanical polishing, cleaning steps making use of a mechanical force are used nowadays. For instance, CMP machines typically used for planarizing or smoothing a semiconductor wafer surface employ a polish slurry containing generally sub-micron sized abrasive particles of a material such as silica or alumina. Large quantities of such particles, along with particles removed from the wafer itself during the polishing operation, may be found on a polished wafer surface. These particles must be removed in a cleaning step or they may drastically affect device yield.
For instance, in the U.S. Pat. No. 5,581,837, disk-shaped objects, e.g. wafers, are transported one at the time between a set of rotating brushes so as to be cleaned thereby. In fact, in the presence of a cleaning liquid, a mechanical force is exerted on the surface of the wafer by means of the rotating brushes to get the wafers cleaned. This cleaning step can be followed subsequently by a rinsing step and a separate drying step. In this drying step, IR lamps are used to performing the drying. It is known that such a technique leaves drying marks on the surface of the substrate.
In an aspect of the invention, a method for removing of contamination particles and a liquid from a surface of a substrate using at least one rotating cleaning pad is disclosed. This rotary cleaning pad is preferably cylindrical shaped and has an outer edge which can contact a surface of a substrate. Particularly, a method is disclosed for the removal of particles and a liquid from at least one surface of a substrate using a plurality of rotating cleaning pads comprising the steps of:
creating a relative linear motion at a predetermined speed between a plurality of wetted rotating cleaning pads and a substrate;
contacting said surface of said substrate at a first edge with each of the wetted rotating cleaning pads of said plurality of wetted rotating cleaning pads each time that during said relative linear motion said wetted rotating cleaning pad arrives at said first edge;
applying a gaseous substance on said surface adjacent to a last wetted rotating cleaning pad, being the last wetted rotating cleaning pad of said plurality of rotating cleaning pads arriving at said first edge, and between said last wetted rotating cleaning pad and said first edge, said gaseous substance being at least partially miscible with said liquid and when mixed with said liquid yielding a mixture having a surface tension being lower than that of said liquid; and
continuing said linear relative motion from said first edge to a second edge, opposite to said first edge, while contacting said surface of said substrate with said plurality of wetted rotating cleaning pads and while supplying said gaseous substance to thereby remove said particles and said liquid from said surface of said substrate. Said gaseous substance can comprise a vaporised substance which is miscible with said liquid and when mixed with said liquid yields a mixture having a surface tension being lower than that of said liquid. A vaporised substance is defined as a mist of finely dispersed liquid droplets of an element or a compound or a mixture of elements or as a vapor. A vapor is defined as the gas phase occurrence of an element or of a compound or of a mixture of elements if the element or compound or mixture should be in the liquid or solid phase at the given temperature and pressure conditions. Thus a vapor can co-exist in one environment with the solid or liquid phase of the element. A vapor is a specific gas phase occurrence of an element or a compound or a mixture of elements. Said gaseous substance can comprise a gas which is miscible with said liquid and when mixed with said liquid yields a mixture having a surface tension being lower than that of said liquid. Said gaseous substance can comprise a mixture of a vaporised substance and a gas, particularly an inert gas like e.g. helium, argon or nitrogen, said mixture being at least partially miscible with said liquid and when mixed with said liquid yielding a mixture having a surface tension being lower than that of said liquid. In an embodiment of the invention, said cleaning pad is moved relative to said substrate while wetting said rotating cleaning pad by supplying liquid to said rotating pad or to said first portion of said surface adjacent to said rotating cleaning pad.
The approach, according to the present invention, is a technique wherein a very sharp liquid-vapor boundary is created on said surface of said substrate adjacent to said last rotating cleaning pad between said last wetted rotating cleaning pad and said first edge.
The relative linear motion can be created in an arbitrary direction including a vertical motion or an horizontal motion. The substrates and/or the cleaning pads can be advanced. Particularly, a transportation belt can be used to advance the substrates.
In another embodiment of the invention, the rotary cleaning pad and particularly the last rotating cleaning pad is cylindrical shaped and has an outer edge which can contact a surface of a substrate. Preferably this outer edge of the cleaning pads is wider than the width of the area of the surface in contact with the cleaning pad. In other words, when contacting said surface, at least a part of the outer edge of the cleaning pad extends beyond the edges of the substrate. Preferably, the rotating cleaning pad is selected such that when wet and when a predetermined pressure is exerted on said cleaning pad a continuous contact area is created between the cleaning pad and the surface of the substrate. More preferably said rotary cleaning pad has a polymeric smooth outer edge. The pad may further comprise a layer underlying said polymeric outer edge, said layer being more compressible than said polymeric outer edge.
In another embodiment of the invention, a method is disclosed for cleaning and removing a liquid from a surface of a substrate using a pair of rotating cleaning pads. Said rotating cleaning pads are arranged in parallel and spaced apart one above the other. A first rotating cleaning pad is used for contacting a first surface of the substrate, a second cleaning pad is used for contacting a second surface of said substrate opposite to said first surface. The rotating cleaning pads and the substrate are handled such that a substrate can be advanced at an adjustable speed through said pair of rotating pads from said first edge of said substrate to said second edge of said substrate, said second edge being opposite to said first edge. Alternatively, the rotating cleaning pads and the substrate can be handled such that said pair of rotating cleaning pads can be advanced at an adjustable speed from said first edge of said substrate to said second edge of said substrate, said second edge being opposite to said first edge.
In another embodiment of the invention, two sets of rotating cleaning pads can be used for removing particles and a liquid from a surface of a substrate. A first set of rotating cleaning pads is used for contacting a first surface of the substrate, said first set comprising a first plurality of wetted rotating cleaning pads, a second set of rotating cleaning pads is used for contacting a second surface of said substrate opposite to said first surface, said second set comprising a second plurality of wetted rotating cleaning pads. The rotating cleaning pads within each set are parallel to and spaced apart one from the other, while each rotating cleaning pad of the first set is spaced apart from and face in face to a rotating cleaning pad of the second set one above the other. In the latter case a gaseous substance is only supplied after and adjacent to the last rotating cleaning pad at each surface to thereby clean and dry said surface.
In another aspect of the invention, a method is disclosed for the removal of particles and a liquid from at least one surface of a substrate using a plurality of rotating cleaning pads comprising the steps of:
creating a relative linear motion at a predetermined speed between a plurality of wetted rotating cleaning pads and a substrate;
contacting said surface of said substrate at a first edge with each of the wetted rotating cleaning pads of said plurality of wetted rotating cleaning pads each time that during said relative linear motion said wetted rotating cleaning pad arrives at said first edge;
locally heating said liquid on said surface adjacent to a last wetted rotating cleaning pad, being the last wetted rotating cleaning pad of said plurality of rotating cleaning pads arriving at said first edge, and between said last wetted rotating cleaning pad and said first edge, to thereby locally reduce the surface tension of said liquid; and
continuing said linear relative motion from said first edge to a second edge of said substrate, opposite to said first edge, while contacting said surface of said substrate with said plurality of wetted rotating cleaning pads and while locally heating said liquid to thereby remove said particles and said liquid from said surface of said substrate. To initiate the drying process, the liquid is locally heated by a heat source to reduce the surface tension to thereby create a sharply defined liquid-ambient boundary on said surface of said substrate adjacent to said last rotating cleaning pad between said last wetted rotating cleaning pad and said first edge. Particularly, the heat source can be a nozzle, movable or not, or a static inlet dispensing a heated gas or a heated vapor or a heated mixture of a vapor and a gas. But also other heat sources can be used such as laser beams or other energetic beams, provided that they can be sufficiently localized.